This invention relates to a keyboard switch of a non-contact type used for a keyboard, more specifically to a keyboard switch of a capacitive-coupling type.
In a capacitive-coupling type keyboard switch (hereinafter referred to as capacitive switch), what is called a capacitor is generally provided with fixed and movable electrodes facing each other at a distance, and a dielectric disposed between these electrodes. The movable electrode is fitted with an operating member whereby the distance between the movable and fixed electrodes can be varied.
It is generally known that the capacitance of a capacitor is in inverse proportion to the distance between the electrodes and varies in proportion to the area of the facing portions of the electrodes where the voltage applied between the electrodes is constant. In the aforesaid conventional capacitive switch, the facing area is fixed, and the distance between the electrodes is variable. Accordingly, the capacitance of the capacitive switch varies with the change of the distance between the movable and fixed electrodes, and the capacitive switch performs switching operation according to the variation of the capacitance. Thus, if the capacitance is higher than a given value, the switch is capacitive-coupled to allow a high-frequency component of AC current to flow. If the capacitance is lower than the given value, on the other hand, the switch is not capacitive-coupled, and is turned off.
Hereupon, in the prior art capacitive switch, the relationship between the change of the depth of depression of the operating member and the change of the capacitance, which depends on the change of the distance between the electrodes, may be represented by a hyperbolic curve, as shown in FIG. 1. According to such relationship, as is evident from FIG. 1, the changing rate of the capacitance is low when the depth of depression is narrow, and increases drastically when the depth of depression exceeds a given value. Thus, the setting range for the operating value of capacitance for the performance of switching operation is quite limited, so that it is very difficult optionally to set the operating value.
For example, if the operating value is set within a range where the rate of change of the capacitance responsive to the change of the depth of depression is high, no great hysteresis can be obtained, and it is hard to prevent an erroneous second time depression of the key due to one's finger's shake.
Namely, the depth of depression of the operating member for shifting the capacitive switch from OFF-state to ON-state (ON-operation depth of depression) and the ON-operation capacitance corresponding thereto are generally set greater than the depth of depression of the control member for shifting the capacitive switch from ON-state to OFF-state (OFF-operation depth of depression) and the OFF-operation capacitance corresponding thereto, respectively. The differences between the ON-operation depth of depression and the ON-operation capacitance corresponding thereto, and the OFF-operation depth of depression and the OFF-operation capacitance corresponding thereto constitute the aforesaid hysteresis. The aforesaid operating value is defined by the ON-operation capacitance.
Since the difference between the ON- and OFF-operation capacitances is normally set constant, the difference between the ON- and OFF-operation depths of depression will be reduced if the operating value for the capacitive switch is shifted as required to a range in which the rate of change of the capacitance responsive to the change of the depth of depression is high. Thus, if the hysteresis is small, the capacitive switch will cause the second time depression or repeated ON-OFF operations attributable to delicate shakings of an operator's finger during the depression, for example. As a result, where the capacitive switch is used in an input unit or an electric typewriter, there will be caused double typing or other awkward effects.